Replace repeated GetBoolArg() calls with Checkpoint::fEnabled variable

set once at init time.

src/checkpoints.h

@@ -24,6 +24,8 @@ namespace Checkpoints
     CBlockIndex* GetLastCheckpoint(const std::map<uint256, CBlockIndex*>& mapBlockIndex);
 
     double GuessVerificationProgress(CBlockIndex *pindex);
+
+    extern bool fEnabled;
 }
 
 #endif